1. Field of the Invention
The present invention relates to a method of forming an ink image by using a printing plate that is used in dry lithographic printing, and to a method of printing the formed image.
2. Description of the Prior Art
Many problems of lithographic printing caused by the use of damping water were solved by the development of dry lithographic printing (see "Balance between Damping Water and Ink" by MUTSUO KOBAYASHI in JAPAN PRINTER, Vol. 69, No. 5, pages 31 to 36, (1986)).
The printing surface of a printing plate used in dry lithographic printing consists of image areas receptive to an ink and non-image areas capable of repelling the ink thereby allowing the ink to adhere selectively t the image areas only. Since the non-image areas require high ink releasability, they are made of a material low in surface energy such as silicone elastomers and fluorine compounds (see "Dry Lithography" by TSUGUO YAMAOKA in JAPAN PRINTER, Vol 60, No. 2, pages 9 to 17 (1977)).
Dry lithographic printing is carried out by supplying an ink to the printing surface of such a printing plate as mentioned above with an ink roller, and transferring the ink adhered on the image areas to an object. In lithographic printing, it is required to use a special ink whose viscoelasticity has been adjusted. The cohesion of the special ink is adjusted to be greater than the adhesion between the ink and the non-image areas so that the ink hardly adheres to the non-image areas.
However, in such conventional dry lithographic printing, maintenance is essential during and after the printing. That is, the ink used in dry lithographic printing will solidify due to oxidative polymerization when left in the air. Therefore, it is required to clean many parts of the printer after the printing operation. On the other hand, since several hours are required for permitting the ink transferred to the object to solidify completely, the objects cannot be placed one over another immediately after the printing, or ink offset would occur.
Further, the cohesion of the in decreases with, to an increase of the temperature. Therefore, when the temperature of the printing surface of the printing plate rises during the printing, the ink adheres to the non-image areas, resulting in scumming on the object. Consequently, it is required to monitor at al times the temperature of the printing surface of the printing plate during the printing.